1. Field of the Invention
This invention relates to printed circuit boards in general, and in particular to multilayer printed circuit boards which have resistors printed on one or more of the board's intermediate layers.
2. Summary of the Prior Art
The density of components and conductor circuits placed on printed circuit boards has been increasing in response to the need to accommodate increasingly complex electronic circuitry in smaller spaces. In addition to accommodating a greater number of conductor paths than heretofore, many printed circuit boards being manufactured today have a greater number of discrete components mounted thereon. Frequently, electronic circuits contained on printed circuit boards require a very large number of resistors to perform various functions, for example, terminating circuits, current limiters or digital attenuators. One method used to increase the density of conductor paths on printed circuit boards has been through the use of multilayer boards in which multiple layers, each containing etched copper circuitry, are sandwiched together. In addition to the method in which individual printed circuit layers are separately fabricated using, for example, a photolithographic process, and then laminated together, there have been uses of thin, screened-on insulating layers between multiple conductor planes to increase the density of conductors on a circuit board. Such an arrangement is described in an article by B. M. Carlisle appearing in the Dec. 11, 1986 issue of Machine Design at pages 101-105. The foregoing article describes the use of polymer thick film (PTF) conductive inks which contain a silver conductive material to produce circuit paths on printed circuit boards by screening the PTF conductors over an insulating layer screened on top of conventionally manufactured etched copper conductors. The article also describes the use of PTF resistive material for producing resistors which may be screened onto the surface of a circuit board. As noted in the foregoing article, however, components which must be connected to the circuits on the board must be mounted on the circuit board's surface and connected to the underlying circuit by means of holes passing from the surface of the circuit board to the plane containing the circuit's conductors. Thus, while the foregoing arrangement may be suitable for providing one or more additional layers of conductor paths, it does not provide any additional space to mount arrays of resistors, especially a large number of resistors, which might be required as integral elements of the circuit's deposited on the printed circuit board. Such resistors must be mounted essentially only on the top layer of the board.
As noted above, in many applications it is desirable to terminate circuits with resistive loads. Heretofore, in most instances such terminations were required to be made by mounting resistors on the surface of a circuit board and connecting the resistors to conductors through holes which passed from the board's surface through one or more conductor planes to the conductor paths to which the resistors are to be connected. Resistors used to accomplish such terminating function could be either conventional carbon composition resistors or resistors screened onto the circuit board's top surface using PTF resistive ink. In the event that surface mounting of the resistors is not suitable, e.g. where resistive terminations are required in a board having many layers and limited surface area, the circuit designer could use a material sold under the name "Ohmega-Ply" by Ohmega Technologies, Inc., Culver City, Calif. The Ohmega-Ply material is composed of a layer of resistive material sandwiched between upper copper conductive layers and lower core material (FR-4 or Polyimide). The Ohmega-Ply material may be used to make resistor-conductor networks using a photolithographic process. See, for example, U.S. Pat. No. 3,691,007 which issued Sept. 12, 1972 and U.S. Pat. No. 3,808,576 which issued Apr. 30, 1974 which provide details of the construction and use of Ohmega-Ply brand materials. The Ohmega-Ply material may be etched using conventional etching processes to produce circuit layers having integral discrete resistors. However, one problem with Ohmega-Ply material which makes it undesirable for use in multilayer printed circuit boards is that it is difficult to produce resistors for a layer whose values substantially differ from each other within the layer e.g., vary by more than one decade. Still another problem is that resistors manufactured from Ohmega-Ply material are not suitable to having their resistance values quickly adjusted by trimming with a laser trimmer because of a tendency of the Ohmega-Ply material to crack due to heating from the laser beam.
U.S. Pat. No. 4,424,251 which issued Jan. 3, 1984 describes a method for manufacturing a thick film multilayer wiring board having thick film resistors in the inner layer. The process described permits the use of thick film resistors manufactured with a binder glass without the binder reacting with a hard glass insulation layer used to cover the conductors on the board. However, as noted in the foregoing patent, an insulating layer comprised of a crystallized glass must be applied over the thick film resistors to prevent reaction between the binder glass in the thick film resistors and the hard glass insulation layer during baking of the hard glass insulation layer. Neither the alumina (ceramic) substrate material, the cermet resistor material, or the hard glass insulation layer are usable in the manufacture of laminated multilayer circuit boards.